1. Field of the Invention
The present invention relates to a quasi-optical coupler with reduced diffraction. This coupler can be used notably at the output of microwave tubes working at high frequency and power, such as gyrotubes. Gyrotrons and gyroklystrons notably belong to this class of tubes.
Tubes of the gyrotron class use the interaction of an electron beam with the component transversal to the axis of propagation of the electron beam of a microwave. This interaction takes place in a cavity in the form of a hollow cylindrical conductor. In a hollow conductor, the distribution of the electrical and magnetic fields is a function, inter alia, of the frequency. A practically lossless propagation of the microwave may take place if the electrical and magnetic fields meet the limit conditions. The tangential component of the electrical field is zero at the walls of the hollow conductor and the magnetic field is the maximum at the walls.
These microwave tubes are generally used in particle accelerator applications or for nuclear fusion. These fields require power values of the order of several megawatts and frequencies in the millimeter or submillimeter ranges.
2. Description of the Prior Art
When the frequency is as high as this, the amplitude of the electrical field, in a cross-section of the hollow conductor along the wall, has a plurality of maximum and minimum values. It then becomes difficult to connect the hollow conductor to a coupler enabling the extraction of the microwave energy from the tube in a mode enabling it to be used easily. Due to the high power, it becomes necessary for the element used as a coupler to be a guide whose diameter is too large in relation to the wavelength of the energy to be extracted. Its diameter represents several wavelengths and the guide is capable of conveying a very large number of modes of varying complexity in addition to the desired mode.
The frequencies and power values necessary for such applications have led the designers of the tubes to make tubes that give microwave power at output in a high-order mode with a complex structure and that convert it into quasi-optical beams. The high-order mode is of the TEm,n or TMm,n type (m and n are whole numbers, n being not zero; they represent respectively the azimuthal and radial indicators or index numbers). Generally, at least one of these indicators is greater than one.
In a quasi-optical beam, it is no longer possible to define any mode and the power density is the maximum in the vicinity of the axis of the beam. It decreases regularly with distance from this axis. In the form of a quasi-optical beam, the microwave energy can be conveyed over large distances with low losses. Mirrors are generally used to guide the quasi-optical beam.
This conversion is generally achieved in a so-called Vlasov-type coupler. It is formed by a waveguide section that receives the microwave energy in a high-order mode at a first end and yields the quasi-optical beam at a second end. The second end has a substantially helical aperture. The energy that comes out of the Vlasov coupler is intercepted by a mirror whose profile is chosen so as to focus this energy or guide it in a determined direction.
The essential limitation of this coupler is its low efficiency: it is of the order of 85%. This is due to the phenomenon of diffraction that occurs along the helical aperture of the waveguide section. The diffracted energy is not intercepted by the mirror and it is not used. It may even be a source of inconvenience if the coupler forms an integral part of a tube. The diffracted energy could get propagated towards the electron gun of the tube or towards the collector and lead to the destruction of certain parts of the tube.